Apparatus for the dewatering of phosphate slimes

ABSTRACT

Method and apparatus for dewatering mill tailings, slimes and slurries, wherein ultra fine particles materially reduce the rate of natural dewatering and hinder their disposal in the mining and processing of phosphate, coal, potash, uranium, talc and the like. The invention is preferably carried out in a closed loop system involving the use of electrokinetic densification and substantially continuous decanting of the supernatant.

This is a continuation of application Ser. No. 860,884, filed 5-8-86,abandoned; which is a division of application Ser. No. 06/734,487, filedMay 16, 1985, U.S. Pat. No. 4,608,179; which is a continuation of Ser.No. 06/543,346, filed Oct. 19, 1983, now abandoned.

The invention relates to the dewatering of waste industrial clays andthe like, permitting their use in landfills and other purposes. At thepresent time such waste is being indefinitely impounded in settlingareas in a colloidal state rendering the impoundment acreage useless.

While the invention is specifically concerned with the mining andprocessing of phosphate rock, it is deemed to have equal application todewatering of many industrial clays and other wastes encountered in theprocessing of coal, potash, uranium, talc and the like.

BACKGROUND OF THE INVENTION

In recent years, the present practice of dewatering phosphate slimes inlarge settling ponds has been the subject of great concern, particularlyin the State of Florida.

More than 50,000 acres of otherwise productive land has been more orless permanently converted to slime disposal areas with 6000 additionalacres being presently converted to that purpose each year. Thisrepresents tremendous losses in land, water and minerals.

STATE OF THE ART

Numerous efforts and proposals have been forthcoming during the lastdecade to improve the rate of dewatering phosphate slimes, prompted byfalling water tables, hazards of impoundment, land use plans andincreasing costs of land and environmental compliance:

(a) The U.S. Bureau of Mines has been active in this field, see reportsRI7892, 8089 and 8498.

(b) Dr. Henry L. Barwood et al of the Florida Phosphate ResearchInstitute has recently published a draft entitled "Phosphate Clay WasteBibliography".

(c) Leslie G. Bromwell et al of the Florida Phosphatic Clays ResearchProject has extensively researched the subject, see pages 541-558 of anarticle entitled "Waste Clay Dewatering and Disposal".

(d) An overview of the subject appears in the November 1977 issue ofWorld Mining, pages 62-64.

(e) See U.S. Pat. Nos. 3,761,239; 4,000,067; 4,107,026; 4,155,233;4,194,960 and 4,330,685.

Since the turn of the century, phosphate slimes, as a waste, have beenhandled in Florida and elsewhere in the same manner. This stems from thefact that the average phosphate processing plant is operated on acontinuous basis, producing in the order of 30,000 gallons of combinedprimary and secondary waste slimes per minute.

The magnitude of the waste involved, having a solids content in theorder of 3.5% as discharged from the plant has prevented any materialdeparture from the original practices of indefinite impoundment inmining cuts and diked ponds.

NATURE OF THE INVENTION

It is an object of the invention to provide a method and apparatus thatlends itself to: (a) materially reducing the size of the settling areasrequired by existing phosphate plants, (b) balancing the dewateringcapacity with the production capacity of the processing plant so as toavoid annual increases in settling areas, (c) improving the rate ofdewatering, preferably without the use of flocculi, thickeners, andchemicals that may have detrimental effects upon recovered water and (d)combining natural dewatering with electrokinetics in such a manner thatthe use of electrical energy including that generated by solar energymay be commercially feasible.

Electrokinetic densification of dredged materials has been heretoforeproposed. See Development of Alternatives for Dewatering DredgedMaterial by Halibarton, pp. 615-631, Solid Waste Materials (supra) andU.S. Pat. No. 4,107,026.

In the illustrated embodiment of the invention, the principle ofelectrokinetics has been adapted to the dewatering of phosphate slimeponds and slimes as discharged from the processing plant to increase itssolids content to the extent required to form a stable landfill eitheras dewatered or when mixed with sand tailings within this narrow range.The economics of the present invention are deemed to compare favorablywith present and projected costs of existing practices in the light ofenvironmental and land use regulations.

In the following description and claims, the expression "closed loop"will be used to describe the mining and processing of phosphate rock andthe like, wherein the dewatering operation is substantially in balancewith plant production of the waste to be dewatered to the end that thearea dedicated to settling purposes is substantially fixed; the areaand/or volume of the settling area being inversely proportional to thespeed of dewatering.

U.S. Pat. No. 4,217,212, issued to the present applicant, dealt with thehistorical problem of waste slimes ponds, rather than the solution, byproviding means for removing slimes from existing impoundments. Thepresent invention eliminates the need for indefinite impoundment andcontrols the densification in such a manner as to permit the condensedslimes to be pumped from the settling area with conventional equipment.

ILLUSTRATED FORMS OF INVENTION

In the attached drawings is disclosed a closed loop system for miningand processing phosphate rock. Two different forms of settling area areshown, as well as several different forms for applying electrokineticsto increase the rate of natural dewatering;

FIG. 1 is a schematic layout of a closed loop system using one form ofsettling area,

FIG. 2 is an enlarged plan view of the three part settling area shown inFIG. 1,

FIG. 3 is a vertical section taken on line III--III of FIG. 2, showingthe use of inflated domes over the settling area,

FIG. 4 is an enlarged plan view of one of the settling ponds shown inFIG. 2 with buoyant electrode supports in position on the pond surface,

FIG. 5 is an enlarged fragmentary vertical section taken on line V--V ofFIG. 4, showing the use of vertically spaced horizontal electrodes,

FIG. 6 is a view similar to FIG. 5 of a modified form of electrodesupport showing horizontally spaced vertical electrodes,

FIG. 7 is a vertical section taken on line VII--VII of FIG. 6,

FIG. 8 is a plan view of another form of settling area,

FIG. 9 is a vertical section taken on line IX--IX of FIG. 8.

It should be understood that the disclosure of the present invention, ofnecessity, has been substantially based upon a small scale experimentalreduction to practice, using fresh plant waste slimes furnished by oneor more phosphate rock processing plants in commercial operation in theState of Florida. However, the solids content, the density of thedewatered slimes and the stable nature of the mixtures of dewateredslimes and sand tailings have all been established following commercialpractice.

An evaluation of the scope and content of the prior art with respect tothe novelty of claims herein presented relative to a commercial closedloop system, requires an examination of the suggestion appearing onpages 557 and 558 of the aforesaid article by Bromwell et al.

As noted on page 542 of the Bromwell et al article, typical settlingponds are presently 400-800 acres in size. Using electrokinetics incombination with natural dewatering as herein proposed, it appearseconomically feasible to reduce the pond size to the order of 25-50acres, or less.

With top decanting immediately on release to remove ions for betterconsolidation contributing 10% or more improvement experienced in thedewatering rate, one of the objects of the present invention relates todepartures made to assure the uniform decanting of clear reusable waterthroughout the entire settling area.

A further object of the invention relates to the advantage of usingelectrokinetics upon fresh plant slimes substantially of the same pH asdischarged from the plant. It appears that the aging of plant slimesresults in an increase in alkalinity detrimental to the dewatering usingelectrokinetics and/or top decanting in combination with naturaldewatering.

A still further feature involves the conception of using buoyant meansfor supporting one or both electrodes between which the current flows inthe application of electrokinetics in the dewatering of slimes. Asdisclosed, these means may take several forms, all of which tend toreduce evaporation and assist in the removal of the dewatered slimes bypumping by reducing coning effects. One advantage of horizontally spacedelectrodes which approach each other as decanting continues resides in areduction in voltage requirements, as the separation diminishes.

Tests conducted with respect to the claimed subject matter clearlyindicate that improvements in the rate of dewatering by direct currentinvolves a substantial expenditure for the electrical energy required.For this reason, the use of solar energy to provide the electricalenergy shows promise particularly in the State of Florida.

Under existing practice, the major cost components are land, dam anddike construction and dike maintenance. To be economically feasible forthe phosphate industry, any alternative method must not greatly exceedsuch costs. However, any proposal that results in the use of less landand reduces the cost of land reclamation has commercial promise.

Turning now to the several forms of the invention illustrated herein forthe purpose of enabling those skilled in the art to place in full scalecommercial practice the principles in method and apparatus of theinvention:

Closed Loop System

Referring to FIGS. 1-3, a schematic layout of a closed loop system isshown in which three earthen settling areas or ponds 10, 11 and 12 areshown into which slimes are preferably flowing by gravity from thephosphate processing plant 13 through the pipe 14. In commercialpractice the pipe 14 may be 48" in diameter and capable of carrying30,000-40,000 gallons of slimes per minute. Pipe 14 is connected with asuitable manifold pipe 15 having remote control valving for selectivelydirecting the slimes into pipes 16, 17 and 18, each having dischargeinto a suitable manifold pipe 19.

Each pond 10, 11 and 12 has a flat earthen bottom 20 upon which isdisposed a network of perforated combination feed and drain pipes 21.Preferably, the pipes 21 are arranged in spaced parallel relation andsized to uniformly flood in sequence the entire bottom area of pond 10,11 and 12. As a result, the level of the slimes in each pond is raisedwith substantially no turbulence to detrimentally affect the promptformation of freed water rising to the surface of the plant slimes beingdischarged into the selected pond.

As the pipes 21 are preferably used to both feed the fresh slimes intothe ponds, as well as to discharge the thickened slimes from the ponds,the sizing and perforating of the pipes 21 must be designed accordingly.

A presettling treatment station 22 for the slimes flowing in pipe 22 isshown in anticipation that reagents may be added to improve thedewatering rate in the ponds in the form of waste acids to control pH,flocculi, thickeners and the like.

To skim the supernatant from the surface of each pond at a ratepreferably substantially conforming to the rate of dewatering, each pond10, 11 and 12 is equipped with a suitable vertically adjustable weir 23discharging into a suitable sump 24; the weirs 23 and sumps 24 beingindicated by lines in FIG. 1.

A suitable pump 25 in pipe 26 connects with sump 24 on the intake sideand with the pipes 27 and 28 on the discharge side; the pipe 27returning the freed water to the plant 13 and the pipe 28 to thepressure pump 29 at the slurry pit 30. Dredged phosphate rock 31 fromthe mining cut 32 is transferred to the plant 13 through the line 33 inthe form of a slurry.

Sand tailings from the plant 13 are transferred through line 34 to themixing station 35, where the tailings are dewatered and mixed with thedewatered slimes being sequentially pumped from ponds 10, 11 and 12. Touse the pipes 21 to remove the thickened slimes from the ponds, suitablemanifolds properly valved and controlled, and indicated by lines 36 inFIG. 1, are connected in sequence with the suction line 37 of the pump38, the discharge side of which is connected by the pipe 39 to themixing station 35. A stable landfill mixture of tailings and thickenedslimes is transferred by pipe 40 and pump 41 to the mining cut 32.

Settling Ponds

As shown in FIGS. 2 and 3, ponds 10, 11 and 12 are preferably designedby sloping earthen sides 42 forming flat top dikes 43, each channeled ortunneled at 44 to provide access to the vertically adjustable weirs 23.The slope of the sides 42 may be varied in practice to the point ofbecoming vertical and cast of concrete.

To secure the maximum benefits of the invention and reduce cost ofconstruction and maintenance of the electrokinetic apparatus, the pondsmay be provided with inflated roof structure 45 of the type in commonuse to enclose tennis courts, playing areas and the like. The perimetersof the structure 45 would be anchored to suitable footings embedded inthe flat tops of the dikes 43. When solar energy is used to provideelectrical energy for improvements in the dewatering rate, a roofstructure capable of supporting suitable solar collectors will be used.

Based upon dewatering rates experimentally obtained in tests conductedand based upon the present invention, with plant discharge of primaryand secondary waste slimes in the order of 30,000 gpm, each pond 10, 11and 12 would approximate eight acres with a depth in the order of twofeet. This would be the minimum sizing of the settling areas based uponmaximum use of electrokinetics and minimum natural dewatering. However,it is anticipated that more surface area with less depth may beadvantageous from the standpoint of voltage requirements resulting fromshallow horizontal electrode separation.

Electrokinetics

Tests run using direct current of adjustable voltage to provide 1 to 2amperes as the electrodes approached each other and substantiallycontinuous decanting of freed water provided slimes of solids contentcapable of forming stable landfill with sand tailings within adewatering time frame of the order of 17 to 26 hours. Best results wereobtained with horizontal parallel electrodes, the upper electrode beingfloated and the lower electrode resting upon the insulated bottom of thetest apparatus.

To equip the ponds with vertically spaced horizontal electrodes, aftergrading and compacting the earthen bottoms 20, suitable plastic film orsheeting 46, as shown in FIG. 4 is rolled out on the bottom in the formof 20' widths with overlapping longitudinal edges. Electrode structure47, such as expanded metal, in commercial roll widths and gauges, isplaced upon the sheeting 46 to hold it in place. Pipes 21 of plastic arethen placed in horizontal spaced parallel arrangement over the entirearea of the bottom 20.

By placing pipes under the lower electrodes, fluid ejected from thesepipes will tend to reduce any accumulation of consolidated material thusreducing voltage requirements.

If the same pipes are to be used to both flood the ponds with plantslimes and to remove the thickened slimes following dewatering, thesizing of the perforations 21' in the pipes 21 will be controlled by theresistance offered by the dewatered slimes to conventional pumping.Preferably the perforations 21' will be located on the horizontal axisof the pipes and staggered on opposite sides along its length. If theponds are sized and dewatered at a rate whereby each pond 10, 11 and 12is refilled approximately ten times each month, the perforations 21'should be in the order of 11/2"-2" in diameter.

Monitoring the Solids Content

By metering the plant slimes during filling of the ponds, as well asmetering the freed water removed across the weirs 23, knowing the solidscontent at plant discharge will readily determine the termination of thedewatering cycle. Ignoring the evaporation that takes place duringdewatering will result in the solids content of the dewatered slimespumped to the mixture station 35 to be on the high side, thus assuringstable landfill.

Other forms of monitoring the solids content of the slimes beingdewatered may be used, such as density meters, etc.

Aside from being electrically conductive, the design, material and gaugeof the electrode 47, resting on the plastic sheeting 46, may take manyforms. Both steel and aluminum commercially available 1/2" mesh expandedsheets have proven satisfactory.

Prior to attempting to pump the dewatered slimes from the settling area,it has been found advantageous to homogenize the slimes by mixing and/orstirring the entire dewatered body. This is for the reasons that thedensity increases from top to bottom during dewatering. It has thefurther advantage of removing an insulating accumulation of materialfrom the electrodes as indicated above.

One practical means for establishing and stabilizing a relativelyuniform density of slimes during pumping is to provide a network ofperforated compressed air plastic pipes 121 adjacent the pipes 21. Theperforations in the air pipes 121 should be very small and directedtoward the pipes 21. To reduce required compressed air capacity to aminimum, the compressed air lines supplying the pipes 121 should be sovalved and controlled that the pond may be sectionally "blown".

In addition to the use of compressed air prior to pumping the dewateredslimes to the mixing station 35, a high volume, high pressure boosterpump 48 may be provided in a reservoired shunt 49 of the gravity slimessupply through pipe 14. Suitable valving and controls may then be usedfor a short period to sectionally flush the pipes 21 with high pressure,high volume plant slimes to prepare the dewatered slimes for pumpingthrough the pipes 21 to the mixing station 35. At the same time theelectrodes on the bottom may be vibrated.

It will be understood that the plant slimes used for flushing would bedrawn from the reservoir of the shunt 49 to prevent "starving" the pump45 during its short period of operation.

Buoyant Electrode Support

FIG. 4 is a fragmentary sectional view of a portion of pond 10 showngreatly out of proportion in order to show the relationship between thestructure supported on the bottom and the buoyant support for the upperelectrode structure.

Slab-like buoyant supports 50 are shown in FIG. 4 floating on the freedwater surface 51 of the slimes, the interface between the supernatantand the colloidal slime suspension being indicated at 53.

Supports 50 may take many forms and may be fabricted from standardcommercial components. As shown, a 4'×8' sheet of 1/4" exterior plywoodprovides a top 54 to which is bonded, on the underside, a 4'×8'×2" slab55 of closed cell foam such as urethane. A 4'×8" sheet of expanded metalproviding the electrode 56 is suitably removably attached to theunderside of the slab 55. Electrode 56 may be of the same gauge andmaterial as the bottom electrode 47. However, other shapes and materialsmay be used for the upper electrode such as lead.

To provide a potential between electrodes 47 and 56, the electrode 47,which extends entirely across the bottom 20, is provided with terminals57. Suitable terminals 58 are in conductive relation to their lower endswith the electrode 56 and extend upwardly through the slab 55 and andtop 54 to removably and conductively engage with clamp action anelectrical conductor 59. Terminals 58 are located adjacent opposite endsof each buoyant support 50 on the centerline thereof.

The conductor 59 extends across the entire width of the pond 10, beingremovably anchored at opposite ends to the dike 43 through a suitableexpansion coil 60 to accommodate the vertical movement of the buoyantsupports 50 during filling, dewatering and pumping. Conductors 59 mayperform the additional functions of an anchor and towline for thesupports 50 in each row.

Attached to the terminals 57 of the electrode 46 are conductors 62which, along with the conductor 59, are connected to sources of directcurrent located on opposite sides of the pond 10. The width of thematerial from which the electrode 46 is fabricated will preferablydetermine the spacing of the terminals 57.

It will be understood from FIG. 4 that substantially the entire surfaceof the ponds 10, 11 and 12 is covered with the buoyant supports 50 witheach row of the supports 50 being connected with a separate conductor59. By having little clearance between adjacent supports 50 in the sameand adjacent rows, excessive relative movement between supports 50 onthe surface of the pond is avoided. The foam slabs 55 will besubstantially protected from abrasion by the plywood tops 54.

Application of Electrokinetics

The arrangement of FIG. 5 is similar to that used in conducting testsduring the development of the invention. Several options are available.Under one option, settling pond 10 may be first filled with plant slimes52 to some selected level 51 before providing an electrical potentialbetween the electrodes 47 and 56, the difference between levels 51 and53 representing natural dewatering during filling.

Under a second option, with the pond empty, the buoyant supports 50 reston the pipes 21 and are insulated thereby from the electrode 47. The "onfill" stage of pond 10 is started floating the supports 50 from theirposition of rest on the pipes 21. At this point a dewatering potentialmay then be established between the electrodes 47 and 56. An interface53 will quickly form between the freed water level 51 and the colloidalsuspension placing the electrode 56 in clear water.

When the dewatering of pond 10 is taking place under the second option,the supports 50 will be raised gently with the rate of rise beingdetermined by the difference between the volume of plant slimes beingdischarged into the pond 10 and the volume of freed water being removedby the weir 23. As the electrodes 47 and 56 will be moving apart,adjustment of the impressed potential may take place to maintain thedesired rate of dewatering.

With the D.C. potential being continuously applied between theelectrodes 47 and 56 for a period in the order of 17-26 hours and theplant slimes continuously flooding the pond 10 at a rate in the order ofB 30,000 gpm, the continuous decanting of freed water will result in thetermination of the "on fill" stage of plant slimes discharge into thepond 10. At this point the level 51 will be in the order of six feet,well under the twenty foot height of the dikes 43.

Under a third option, the use of electric current to improve thedewatering rate would be confined to "off-peak" periods to takeadvantage of lower electrical rates. If this option is selected, thelevel 51 at the end of 17-26 hours of "on fill" stage would be in theorder of 10-15 feet depending upon the length of the "off peak" period.

Many other options of operation are available to balance the dewateringof plant slimes with the plant production of such slimes.

"On Dewatering" Stage

Following the "on fill" stage, pond 10 is at the "on dewatering" stage.If the average solids content of the dewatered slimes in pond 10 isbelow that required to form a stable landfill with sand tailings,further dewatering will take place by natural dewatering or byelectrokinetics or a combination thereof.

The "on dewatering" stage in pond 10 will be terminated as soon as thedesired density is obtained and may be omitted completely under certainoperating conditions. Preferably the next step is to flush the pipes 21with plant slimes for a short period using the booster pump 48. Ifdesired, this step may be carried out in combination with the use ofcompressed air sectionally blown through the pipes 210. This combinedaction of back flushing the pipes 21 and use of compressed air willimprove the uniformity of density of the slimes as well as thepumpability.

The final step performed in pond 10 is the "on removal" stage duringwhich the thickened slimes will be pumped from the pond 10 to the mixingstation 35 and the mixture of slimes and tailings discharged into themining cut 32.

"On Removal" Stage

The "on removal" stage in pond 10 begins with the use of the pipes 21 toremove the thickened slimes by the pump 38. As the level 51 in pond 10is lowered by pump 38, the buoyant supports 50 will follow the level 51until the electrodes 56 come to rest on the pipes 21.

It has been observed in tests that the flat undersurface and floatingweight of the slabs 55 acts to reduce the tendency of the slimes to"cone" adjacent the perforations 21' under the suction of the pump 38.With the supports 50 resting on the pipes 21 the pumping operation isterminated and pond 10 is now in condition to be inspected and returnedto the "on fill" stage.

In practice, the level 51 is lowered to a point floating the supports 50slightly above the pipes 21. This enables the rows or supports 50extending across the narrow width of the pond 10 to be readily shiftedhorizontally relative to each other. Thus an inspector may walk betweenrows of floating supports 50 on the electrode 47 supported by the bottom20. During major component replacement, the floating individual rows ofsupports 50 may be retrieved from the dikes 43.

Adjustable Weirs

It will be appreciated that the level 51 will be affected by the plantslimes 52 flowing into the pond 10 through pipes 21, by the decanting ofthe freed water and by the thickened slimes removal to the mixingstation 35. During all of the stages taking place in pond 10, there willbe substantial freed water above the interface 53 except for a shortperiod at the start of the "on removal" stage.

As only clear water is intended to flow over the weir 23, the height ofthe weir 23 must be continuously adjusted relative to the interface 53.A suitable sonic, laser reflective sensor or the like may be used tocontrol the mechanism kregulating the height of the weir 23. Preferablythe operation of the weir 23 will be programmed to decant clear waterfrom above the interface 53 at all times except just prior and duringthe period the thickened slimes are being "blown" by compressed air inpreparation for removal.

Sequencing of Ponds

While the three ponds 10, 11 and 12 illustrate the settling area devotedto dewatering of plant slimes in my closed loop system, the maximumdewatering rate of the system will permit the use of only two of thethree ponds. This will enable any one of the three or more ponds of thesettling area to be out of service for repairs, replacement ofcomponents and the like.

Depending upon the sizing of the settling area, and the capacity of themeans for improving the rate of natural dewatering, the sequence ofoperations will require that the "on fill" stage of the pond 11, forexample, directly follows the termination of the "on fill" stage of pond10.

As pond 11 goes through its filling and dewatering stages, pumping willhave been completed in pond 10 in advance of the termination of the "onfill" stage in pond 11 thus enabling the discharge of plant slimes to beswitched back to pond 10.

Obviously, the use of only two of the three ponds 10, 11 and 12 in thesequence of operation of the closed loop system, will increase theamount of electrical energy used in the form of direct current. It willalso require that the dewatering of the slimes takes place during the"on fill" stage to permit the use of pipes 21 for both input and outputmovement of the slimes unless the pumping time of the thickened slimesis less than the "on fill" time. In that event, additional dewateringmay be sandwiched between the termination of the "on fill" stage and thestart of the pumping operation.

Natural Dewatering

It is anticipated that in actual commercial practice of the presentinvention, the settling area may embrace four ponds, in lieu of three,in order to avoid the necessity of improving the natural dewatering rateduring the "on fill" stage by electrokinetics.

With four ponds and the elimination of electrokinetics, it isanticipated that the presettling treatment station 22 will come intoplay for the addition of reagents capable of improving the rate ofnatural dewatering.

As indicated herein, continuous top decanting of the freed water hasprovided an observed improvement in the natural dewatering rate in theorder of 10%.

Vertical Electrodes

A modification of the electrode arrangement of FIG. 5 is shown in FIGS.6 and 7 in which spaced vertical electrodes are used in lieu ofhorizontal electrodes in the application of electrokinetics to my closedloop system. It has for one of its advantages, the elimination of thelaminated association of the plastic film or sheeting, and the electrode47. Comparative tests indicate that this modification may not provide asmuch improvement in the dewatering rate as the horizontal electrodes ofthe form of FIG. 5 for the same expenditure of electrical energy.Contributing to this difference in performance is believed to be thefact that in the form of FIG. 5, gravity reinforces separation and thespacing of the electrodes is reduced when the level 51 is lowered,whereas in the form of FIGS. 6 and 7, the spacing of the electrodesremains the same throughout the application of direct current to theslimes.

As shown in FIGS. 6 and 7, the buoyant electrode supports 50' may befabricated from 4'×8' slabs of 4" foam. Heavy duty plastic film strips64, in the order of 6 to 10 mils of a continuous length, approximatingthe narrow width of a settling pond, is bonded to the oppositelongitudinal vertical sides 65 of the supports 50' with their upperedges 66 bent over and extending along at least a portion of the topsurface 67 of the supports 50' and bonded to the surface 67.

The film strips 64 have portions depending below the sides 65 to whichare bonded continuous lengths of electrode members 68 in vertical spacedrelations. Electrode members 68 are coextensive with the strips 64 andterminate adjacent opposite sides of the settling pond in conductiveterminals 69. Flexible conductors 70 extend between the terminals 69 anda suitable source of direct current. An extension coil 71 is provided byeach conductor 70 to accommodate the range of vertical movement of thesupports 50' on the surface 51 of the slimes 52.

Film strips 64 act to reinforce and protect the sides 65 of the supports50'. They also act to slightly separate the adjacent ends 72 of thesupports 50', as well as to connect the supports 50' of the same rowextending across the narrow width of the pond. In addition, thedepending portions 64' of the strips 64 provide the support for theelectrode members 68 is spaced parallel relation; the electrode members68 collectively defining, on opposite sides of the supports 50', thepositive and negative electrodes 73 and 74.

Spaced by the vertical sides 65 of the supports 50', the weight of theelectrode members 68 tends to vertically position the electrodes 73 and74. By being bonded on the inside of the depending portions 64', themembers 68 collectively defining the electrodes 73 and 74 are insulatedfrom the depending electrodes of adjacent rows of supports 50'. Also, asindicated in FIG. 7 by plus and minus signs, the depending electrodesmost adjacent the electrodes of adjacent rows of supports 50' may be ofthe same polarity.

The electrode members 68 may be of milled edge flat aluminum stock.Preferably, the members 68, in addition to being spaced, are connectedonly by the flexible film to which they are bonded. This allows theelectrodes 73 and 74 to collapse in a vertical direction into an orderlypile during pumping when the electrodes 73 and 74 come to rest upon thepipes 21. Thus the advantage of the form of FIG. 5 is retained in regardto the action of the flat underside of supports 50' to reduce "coning"of the thickened slime during removal.

To install the supports 50' on the surface of the pond, it isanticipated that the rows of suppors 50' will be most convenientlyassembled on the edge of the pond and floated into position as a longcontinuous component of a length approximating that of the pond bottom20.

Aspects of Vertical Electrodes

Aside from their support of the members 68, defining the electrodes 73and 74, the depending portions 64' have several other functions: (a)they provide surface channels for the freed water extending across thepond which are relatively undisturbed by wind or wave action; (b) theyconstitute vertical baffles depending from the level 51 to reduceturbidity between the clear water and the interface 53; (c) they channelthe flow of the freed wafter adjacent the dikes 43 on its movementtoward and across the weir to effectively skim the supernatant above theinterface 53; and (d) they reduce the effect of air movement over thepond to modify the level of the pond surface.

Buoyant Decanting

In the use of testing equipment in the reduction to experimentalpractice of the present invention, it was found convenient to syphon thesupernatant. This was done by supporting the intake of the syphon fromthe buoyant electrode support. As the level 51 moved up and down byintake of slimes or decanting of freed water, the syphon intake wouldretain its same relation to the level 51.

FIG. 7 shows each row of supports 50' equipped with a perforated plasticsyphon pipe 75 substantially coextensive with the electrodes 73 and 74.The pipe 75 is bonded along one side to one of the strips 64 of eachsupport 50'.

Pipes 75 of adjacent rows of supports 50' may be flexibly connected atopposite ends to manifold pipes located along the sides of the pond andconnected to a suitable suction pump for delivery of the freed water tothe plant.

As shown in FIG. 7, the pipes between adjacent rows of supports 50' actas bumpers between the rows protecting the supports 50' from abrasion.If desired, the manifold suction pipes on opposite sides of the pond andextending parallel to the longitudinal axis of the pond, to which thepipes 75 connect, may also be perforated and floated to function in thesame manner as the pipes 75. With such an arrangement, some or all ofthe freed water flowing in the channels defined between the strips 64,will be removed by the manifold pipes to reduce or eliminate the needfor the weir 23.

Pilot Plant

Anticipating that a pilot plant operation may be required to demonstratethe commercial use of the present invention, in FIGS. 8 and 9 is shown atank 76 having a diameter of 64' and a height of 7' of which 2' is theheight of the outer wall of the filter bed 77 at the vertical wall 78. Aconical wall 79 defines the upper wall of the bed 77 and the bottom ofthe tank 76 and extends from the wall 78 to a centrally located opening79' in which a selector valve and pump intake assembly 79" is located.

Vertical radial walls 80 divide the tank 76 into three separate settlingareas or sections 81, 82 and 83, corresponding in purpose to the ponds10, 11 and 12 of FIGS. 1 and 2. Fresh slimes from the plant areselectively discharged into the three sections through a selectivelyrotated discharge 84 to fill the sections 81, 82 and 83 in sequence.

To decant the freed water as it is formed on the surface of the plantslimes discharged into the tank 76, tubular weirs 85 are adjustable towithdraw through the lower pipe sections 86 only the freed supernatant.Sections 86 open through and they are vertically supported from thebottom 79 and electrically insulated therefrom. The supernatant flowsdownwardly in the section 86 into the filter bed 77 from which the clearwater is removed by the pump 87 and returned to the plant by the pipe88.

Means for providing the weirs 85 with vertically adjustable decantingpositions may take many forms. For example, the upper section 87 may bein the form of a split sleeve, which, when rotated, will progressivelyexpose vertically arranged openings in the section 86; the openingsbeing disposed in a spiral path.

To provide the dewatering areas of the tank 76 with electrokineticcapability, the inner surface of the wall 78 is insulated from thebottom 79 as well as from the pipe 84' extending to the nozzle 84.Likewise, the vertical radial walls 80 are insulated from the conicalbottom 79 and pipe 84'. With the walls 78 and 80 acting as a positiveelectrode and the weirs 85 including the sections 86 acting as anegative electrode, direct current may flow between these verticalelectrodes through the plant slimes sequentially discharged into thetank sections 81, 82 and 83.

Pilot Plant Sequence

A sequence of operation of the pilot plant structure of FIGS. 8 and 9may be as follows: fresh plant slimes are directed into section 81 withthe nozzle 84 in the position shown. Suitable sonic sensors will haveadjusted the weir 85 in section 81 to shut off all drainage into thefilter bed 77. With the section 81 filled to a selected level, thenozzle 84 is rotated to initiate the filling of section 82. At the sametime the weir 85 in section 81 has been adjusted to decant thesupernatant above the interface into the bed 77 to be returned to theprocessing plant and/or the slurry pit by the pump 87. At the same time,if desired, direct current is flowing between the walls 78 and 80, andthe weirs 85 associated with the section 81.

By the time section 82 has been filled, dewatering in section 81 hasthickened the slimes in section to the desired solids content. At thesame time, section 83 is placed "on fill" stage, the "on removal " stageis initiated in section 81. To that end, the sectional valve of assembly79" is opened into the interior of section 81 and the thickened slimesof section 81 are pumped out through the pipe 89. Slope of the conicalbottom 79, vibration of the bottom 79, as well as the application ofcompressed air and slushing, will aid in the removal of the dewateredslimes.

When the "on fill" stage in section 83 has been completed, the "onremoval" stage in section 81 has been completed a short time priorthereto. Preferably, the initial flow of plant slimes into the section81 to repeat the sequence of operation is used to flush the thickenedslimes still remaining in section 81 down the slope of the bottom 79toward the opening 79', regulated by the assembly 79".

As in the case of the other forms of the invention, dewatering anddecanting preferably takes place during the "on dewatering" and "onremoval" stages. By separately flooding the bottom 79 of each section81, 82 and 83 in lieu of the single nozzle 84, decanting of thesupernatant may take place during the "on fill" stage. If the sectionsare to be separately flooded, the discharge of the plant slimes will bethrough the wall 78 adjacent the upper edge of the conical bottom 79 toflush the slimes remaining from the "on removal" stage toward theopening 79'.

To balance the dewatering phase of each section 81, 82 and 83 with theamount of plant slimes being directed into the pilot plant fordewatering, the duration and voltage of the electrokinetics beingapplied may be adjusted.

Electrode Replacement

The use of electrokinetics to improve the rate of dewatering willrequire replacement of the electrodes from time to time. When one orboth electrodes is buoyantly supported on the surface of the slimesbeing dewatered, it is anticiptated that the buoyant support structurewill not be replaced as frequently as the electrodes. For this reasonthe electrodes should be removably attached to the buoyant supportstructure.

With reference to the form of FIGS. 6 and 7, the replacement of theelectrodes 73 and 74 would involve severing the depending portions ofthe strips 64 from the portions bonded to the supports 50'. Thereplacement electrodes would then be removably attached to the supports50' in substantially the same location as the original dependingportions.

Should it be found feasible to enlarge the pilot plant of FIGS. 8 and 9,and place the same in commercial use, surfaces which function aselectrodes as disclosed in reference to the pilot plant, will bereplaced with suitable removable laminate.

Flooding Impoundments

In lieu of using the pipes 21 to both flood the ponds with slimes and toremove the dewatered slimes, the pipes 21 may be used only to remove thedewatered slimes. In this event the fresh plant slimes may be dischargedat one or more points along both longitudinal sides of the ponds withthe buoyant electrode supports 50' acting to smooth out the turbulenceof the discharge into the pond. The descending electrodes of the form ofFIGS. 6 and 7 will materially assist such action.

To enable early dewatering and decanting of the slimes, the discharge ofplant slimes into the pond is preferably adjacent the bottom 20 to avoidturbidity between the level 51 and the interface 53.

SUMMARY

From the description of the operation of the embodiments described aboveit will be appreciated that an effective system for dewatering slimes isachieved by the invention. The improvements resulting from the inventionresult from the described relationship of the components which permit anumber of operational features to occur which have proven to augment theprocess. For instance, it is highly desirable that the upper electrodesbe of a negative potential, while the positive polarity occurs at thelower regions of the treatment volume. The flow of water is toward thenegative electrode and the resulting hydrogen that is produced byelectrolysis occurs in the relatively clear water adjacent the negativeelectrodes. Thus, the creation of hydrogen, and the associated bubbles,do not produce agitation at the lower regions. The effect of thepositive potential at the bottom of the treatment volume permits gravityto assist in the dewatering process as natural precipation of the solidsoccurs.

With the apparatus of the invention it is possible to intermittentlyapply the voltage to the electrodes to achieve a "coasting" effect. Forinstance, the voltage may be applied to the electrodes for ten minutes,and then terminated for fifty minutes. Such operation reduces theelectricity requirements and yet permits an effective process ofdewatering to occur.

It is also to be appreciated that the described dewatering system onlyrequires low voltages and the voltage requirements are inverselyproportioned to the treatment time necessary. Thus, it is possible togenerate the electricity required by the process by solar means, and asthe need for the process is primarily in Southern States such as Floridawhich enjoy high percentages of direct sunlight during most days, solarelectric generation to provide energy for the process is feasible.

It is appreciated that various modifications to the basic inventiveconcepts may be apparent to those skilled in the art without departingfrom the scope of the invention, and the invention is to be defined onlyby the language of the following claims.

I claim:
 1. In a closed loop system for the dewatering of slimes havingfill, dewatering and removal stages, an impoundment component for thesequential stages of storage, dewatering and removal of fresh plantcolloidal compositions for landfill and other purposes, means fordischarging the composition into the impoundment to raise the level inthe impoundment, floating buoyant means supported on and moving with thelevel of the composition during the fill, dewatering and removal stagesof the composition within and being processed in the impoundment, saidbuoyant means reducing the surface exposure of the composition to theenvironment, and electrokinetic dewatering means supported by saidbuoyant means and moving therewith as a unit to improve the naturaldewatering rate when activated.
 2. In a closed loop system as defined inclaim 1 wherein said buoyant means supports electrode means of the samepotential.
 3. In a closed loop system as defined in claim 1 wherein saidbuoyant means supports electrode means of different potentials insubstantially fixed relation.
 4. In a closed loop system as defined inclaim 2, electrode means of different potential supported in fixedrelation within the impoundment and insulated from the electrode meansof said buoyant means.
 5. In a closed loop system as defined in claim 3,said electrode means depending in spaced relation from said buoyantmeans.
 6. In a closed loop system as defined in claim 5, said electrodemeans being collapsible upon engagement with a non-fluid surface of saidimpoundment.
 7. In a closed loop system as defined in claim 1, removalmeans located adjacent the bottom of the impoundment for the removal ofthe dewatered composition by suction during the removal stage, and meanssupporting at least some of said buoyant means relative to said removalmeans to reduce "coning" of the composition by the suction action duringthe removal stage.